Various electronic apparatuses such as a mobile phone and a mobile information terminal device (PDA) have been widely used, and it has been demanded to further reduce the size and the weight of the electronic apparatuses and to achieve their longer lives. Accordingly, as an electric power source, a battery, in particular, a compact and light-weight secondary battery having high energy density has been developed.
In these days, application of the secondary battery is not limited to the above-described electronic apparatuses, and various applications have been considered. Representative examples of such various applications may include a battery pack attachably and detachably mounted on the electronic apparatuses or the like, an electric vehicle such as an electric automobile, an electric power storage system such as a home electric power server, and an electric power tool such as an electric drill.
There have been proposed secondary batteries that utilize various charge and discharge principles in order to obtain battery capacity. In particular, attention has been paid to a secondary battery that utilizes insertion and extraction of an electrode reactant, because higher energy density is achieved thereby, compared to energy density in a battery such as a lead battery or a nickel-cadmium battery.
A secondary battery includes a cathode, an anode, and electrolytic solution. The anode includes an active material (anode active material) that is capable of inserting and extracting an electrode reactant. As the anode active material, a carbon material such as graphite has been widely used; however, it has been considered to use silicon therefor recently in order to further improve a battery capacity. One reason for this is because a theoretical capacity (4199 mAh/g) of silicon is remarkably larger than a theoretical capacity (372 mAh/g) of graphite, and it is therefore expected to largely improve the battery capacity thereby. In this case, tin is also considered promising that has a high theoretical capacity as with silicon.
However, because silicon or the like expands and contracts radically at the time of charge and discharge, the anode active material easily cracks mainly in the vicinity of a surface layer thereof. Crack of the anode active material causes a new highly-reactive surface (active surface) to be produced, which results in increase in the surface area (reactive area) of the anode active material. Accordingly, a decomposition reaction of electrolytic solution is caused in the new surface, and the electrolytic solution is used in order to form a coating film derived from the electrolytic solution on the new surface. This results in easier decrease in battery characteristics such as cycle characteristics.
Accordingly, various considerations have been made on a configuration of the secondary battery in order to improve the battery characteristics. Specifically, in order to control variation in volume at the time of charge and discharge and to improve electric conductivity, a surface of a composite particle of silicon and metal is covered with a carbon nanotube (for example, see Patent Literature 1). This carbon nanotube is formed by thermal decomposition and carbonization of hydrocarbon gas, and grows using a metal component in the composite particle as a catalyst.